completeMultiFileDownload captured an index via firstIndex, then awaited a
detached file-size calculation. Another concurrently-completing download could
mutate activeDownloads during that suspension, leaving the index stale and
crashing in Array.remove(at:) with an out-of-bounds index. Remove the download
by identity instead.
Fixes a SIGTRAP seen in TestFlight build 261 (5 reports).
The background URLSession could be in an invalid state when downloadTask(with:)
is called, because invalidateAndCancel() is asynchronous internally. This adds
an ObjC exception handler to catch NSExceptions from CFNetwork, nil guards on
the session, and safer session lifecycle management (nil after invalidation,
finishTasksAndInvalidate for cellular toggle).
